The present invention relates to an optical recording medium and an optical memory device capable of reading or recording in real time digital information, such as computer data, facsimile signals and digital audio signals, by an optical beam such as a laser beam. More specifically, the invention relates to a high density phase change optical disk and a high density read-only optical disk.
As information diversification has progressed in recent years, there are growing demands for a rewritable optical disk that allows a user to record or rewrite information. Further, an increase in the amount of information has called for large-capacity rewritable optical disks. Under these circumstances, a variety of research organizations are actively engaged in studies for increasing the recording density of an optical disk. These studies include those on a method for reducing the size of a recording mark reducing the apparent spot diameter of an information reading laser beam by reducing the wavelength of a recording laser beam or by increasing the numerical aperture (NA) of a focusing lens to increase the recording density, as described in the Nikkei Electronics, Vol. 521, page 92 (1991). A super-resolution reading method using a super-resolution layer of organic pigments is reported in the proceedings of lectures in the Japan Society of Applied Physics, page 1000, 19p-K-5 (autumn, 1994).
The method of increasing the recording density by using the super-resolution layer to reduce the apparent spot diameter of an information reading laser beam is important because it can easily be combined with other recording density enhancing methods.
The object of the invention is to provide an optical recording medium and an optical memory device which have a large capacity and can perform a recording or reading operation a large number of times.
In the present invention, the recording medium has a construction in which a super-resolution layer (hereinafter called a mask layer) whose refractive index is changed in a part of a light spot by a laser beam of 2 to 4 mW is sandwiched by inorganic protective layers and in which the difference in the amount of reflected light between a crystalline state of the film and an amorphous state of the film is so small in either the region where the refractive index has changed or other remaining regions that the playback signals are difficult to identify.
That is,
(1) An optical recording medium that enables at least information to be read out by irradiation of a light, comprises: a mask layer containing at least one of elements in groups Ib, IIb, IIIa to VIIa, and VIII in 10 to 40 atomic percent and also containing oxygen; wherein when the read operation is performed at a reading laser power of 1 mW by using a lens with a predetermined wavelength and a predetermined numerical aperture and with a light spot focused to nearly a diffraction limit, the difference between the amount of reflected light from recording marks and the amount of reflected light from spaces between the recording marks is 25 or less, with the amount of reflected light from the spaces taken as 100, and there is a laser power in a reading laser power range of 2 to 4 mW that causes the difference between the amount of reflected light from recording marks and the amount of reflected light from spaces between the recording marks to be 30 or more, with the amount of reflected light from the spaces taken as 100. Alternatively,
(2) An optical recording medium that enables at least information to be read out by irradiation of a light, comprises: a mask layer containing at least one of elements in groups Ib, IIb, IIIa to VIIa, and VIII in 10 to 40 atomic percent and also containing oxygen; wherein when the read operation is performed by using a lens with a predetermined wavelength and a predetermined numerical aperture and with a light spot focused to nearly a diffraction limit, there is a laser power in a reading laser power range of 2 to 4 mW that causes the difference between the amount of reflected light from recording marks and the amount of reflected light from spaces between the recording marks to be 25 or less, with the amount of reflected light from the spaces taken as 100, and in a lower laser power range there is a laser power that causes the difference between the amount of reflected light from recording marks and the amount of reflected light from spaces between the recording marks to be 30 or more, with the amount of reflected light from the spaces taken as 100. More preferably, the mask layer further includes silicon.
The measurement of the reflected light is taken at the recording marks and spaces that are three times or more longer than the shortest recording mark.
More preferably, the mask layer uses an inorganic film containing at least one of elements, such as Co, O and Si, in 30 atomic percent or more to equivalently reduce the spot diameter of the laser beam. The conventional organic super-resolution layer has a property such that, when applied with a laser beam with an intensity in excess of a threshold value for a predetermined duration, the layer loses molecules in a ground state and thus can no longer absorb light (saturation of absorption). On the other hand, the mask layer of this invention has a property such that when the mask layer is applied with a laser beam, the refractive index changes without changing the absorption coefficient. To take full advantage of this property, a laminated film is provided with the characteristics described in (1) and (2) above. As a result, in a recording medium of the type (1), when the normal reading laser power of about 1 mW is used, the difference in the amount of reflected light between the crystalline state and the amorphous state of the phase change recording film is small and therefore the reproduced signal small. As the reading laser power is increased, the refractive index of the mask layer changes in a part of the light spot to produce a large playback signal. It should be noted, however, that even at this time the remaining region in the light spot has a small difference in the amount of reflected light or, in other words, masked. When the reading laser power is at around 1 mW, the amounts of reflected light from both states may be low. However, it is not required that the reflected light amounts from both states be low. As described above, when information on the track is read out, only the information on those portions where the refractive index is changed can be read. This produces the same effect as when the information is read by using a small light spot (super-resolution effect). At this time, if the mask layer is provided between the substrate with at least its surface formed of an organic material and the recording film, the super-resolution effect can be obtained at a low reading laser power. However, because the mask layer absorbs light, the recording power becomes high. In that case, it is preferred that the mask layer be sandwiched between other inorganic protective layers to avoid unwanted influences from substrate deformations caused by heat generated in the mask layer. In another structure, the mask layer may, for example, be formed between an aluminum alloy reflective layer and a ZnS-based inorganic protective layer, which is on the reflective layer side of the recording film. The mask layer, when interposed between inorganic protective layers, has an increased mechanical strength and thus undergoes a minimal deformation even after a large number of rewriting operations. When the mask layer is formed on the reflective layer side of the recording film, the heat generated in the mask layer can be released toward the reflective layer, minimizing thermal damages to the mask layer. This reduces deformations and structural breaks of the layer due to heat during the recording operation. At this time, a reflective layer with a large thermal conductivity should preferably be used because it helps release heat more quickly. It is more preferred that the mask layer be formed in contact with the reflective layer for an increased heat dissipation effect.
In a recording medium of the type (2), when a reading laser power of 2 to 4 mW is used, the refractive index of the mask layer changes in a part of the light spot, in which the difference in the amount of reflected light between the crystalline state and the amorphous state of the phase change recording film is small and therefore the reproduced signal small. That is, the area where the refractive index has changed is masked. In the remaining area in the light spot a large reproduced signal can be obtained, realizing the super-resolution reading.
The area in which a refractive index change occurs is an area where the accumulated amount of light received is large or an area where the temperature rises due to the applied light. An example of such an area is illustrated in FIG. 1.
Of the recording mediums (1) and (2), the type (1) is more advantageous in narrowing the track pitch because the area where the refractive index changes constitutes an unmasked region (called aperture) so that the expanse in a direction perpendicular to the recording track in the unmasked region is narrow and the signal from the adjacent track is not easily seen.
The reading laser power of 2 to 4 mW in the recording mediums (1) and (2) corresponds, in the ordinary optical system, to an average power density of 3 mW/xcexcm2 to 6 mW/xcexcm2 in a range of up to 1/e2 of a peak power density.
In this invention, the combined use of a recording film and a mask layer whose refractive index is changed by a laser power, which is higher than the average reading power and lower than the recording power (high-level power), can execute good recording and playback operations.
The present invention further has the following features.
(3) Inorganic protective layers are provided on both sides of the mask layer.
(4) The mask layer is formed between the recording film and the reflective layer.
(5) The mask layer is formed between the recording film and the substrate.
(6) The mask layer contains at least one of elements, Co, Fe, Ni, Cu and Ag, in 10 to 40 atomic percent.
(7) Of the constitutional components making up the mask layer, one or more components whose melting point is 100xc2x0 C. higher than the melting point of the recording film occupy 80% or more of the total number of atoms of the mask layer.
(8) Of the constitutional components making up the mask layer, one or more components whose melting point is 1000xc2x0 C. or higher occupy 80% or more of the total number of atoms of the mask layer.
(9) Information can be recorded by using 2.5 to 8 times the laser power that produces a difference in the amount of reflected light of 20 or less.
(10) Of the constitutional components making up the recording film, one or more components with a composition of Gexe2x80x94Sbxe2x80x94Te or Inxe2x80x94Sbxe2x80x94Te occupy 80% or more of the total number of atoms of the record film.
(11) The optical memory device can read information from the optical recording medium described in (1) or (2) and which has a means for setting the laser power at a high level that produces a large playback signal during the read operation and for setting the laser power at a lower level only when performing the autofocusing and tracking operations.
In this invention, the reading beam may use a pulse beam rather than a continuous light (DC light).
Among the preferable recording films that can be used in this invention are a hole type recording film, a high melting point crystalline-amorphous phase change optical recording film capable of high-speed recording and erasure, a recording film utilizing an amorphous-amorphous phase change, and a crystalline-crystalline phase change record film utilizing changes in crystal system and crystal grain diameter. Other types of recording film may also be used.
By forming the mask layer of an inorganic material and interposing the mask layer between layers of inorganic materials, the invention increases the number of times that the rewriting operations can be carried out. This is because the inorganic protective layers on both sides of the mask layer improve the mechanical strength and because one of the inorganic protective layers interposed between the substrate and the mask layer helps prevent thermal deformations of the substrate. Setting the mask layer and the reflective layer in a closer relationship can release the heat generated in the recording film and the mask layer to the reflective film quickly. The reflective film in this case should preferably be formed of such materials as aluminum alloy with a high thermal conductivity because a greater heat releasing effect can be produced.
The present invention is particularly suited for a phase change type optical disk capable of what is generally called a one-beam overwrite operation that records, by the application of a laser beam, new information while at the same time erasing existing information. The invention is also suited for a write once type disk that cannot be overwritten. This invention is particularly effective for chalcogenide (e.g., a recording film made mainly of Inxe2x80x94Se, Gexe2x80x94Sbxe2x80x94Te or Inxe2x80x94Sbxe2x80x94Te) which contains at least one of chemical elements, Te, Se and S, in 30 to 85 atomic percent, and also for a recording film made mainly of Inxe2x80x94Sb. Other recording mediums with different recording principles may also be used.
In addition to the disk, the recording medium of this invention may take other forms such as tape and card.